As techniques for a circuit board including a wiring layer therein, the following techniques are known. According to one of the known techniques, a through hole formed in a printed circuit board is plated with a conductive material, a conductive pad is formed around the through hole on one surface or both surfaces of the printed circuit board, and the conductive pad outside the through hole on the printed circuit board is removed by back drilling.
Another known technique is to decrease the length of a conductive stub by drilling at least a part of a through hole. Still another known technique is to shorten a stub by making a through hole for a signal pin shorter than another through hole by back drilling.
In order to mechanically and electrically connect a connector to a circuit board, used is a via including a through hole formed in the circuit board and a conductor that covers an inner wall surface of the through hole. More specifically, a connector pin of the connector is inserted into the through hole to contact with the conductor that covers the inner wall surface of the through hole. The conductor that covers the inner wall surface of the through hole is connected to a signal wire formed inside the circuit board, and the signal wire is electrically connected to the connector pin through the via.
A part of the via may form a wire branch called a stub, and may cause a negative effect on a signal flowing in the signal wire. That is, when a signal reaches a branch point of the stub from the signal wire, the signal flowing in the signal wire is divided into two signal components, one of which flows to the stub, is reflected on an end of the stub, and returns to the branch point again. As a result, the signal flowing in the signal wire may attenuate in some cases at the branch point due to interference between the signal flowing in the signal wire and the signal reflected on the stub end.
Back drilling of inserting a drill from a back surface of a circuit board at a position where a via is formed, and cutting and removing a stub is known as a method for removing a stub. However, a reflection loss caused by the stub increases as the signal transmission rate becomes higher, and high-speed transmission exceeding 25 Gbps (bps: bits per second) may not be achieved merely by performing the conventional back drilling.
The followings are reference documents.                [Document 1] Japanese Laid-open Patent Publication No. 2002-110274,        [Document 2] Japanese Laid-open Patent Publication No. 2012-248653, and        [Document 3] Japanese National publication of International Patent Application No. 2010-537402.        